Re: [PATCH 2/3] block layer varlen-cdb

[Benny Halevy <bhalevy@panasas.com>]

On Apr. 03, 2008, 19:43 +0300, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> On Tue, 2008-03-25 at 18:32 +0200, Boaz Harrosh wrote:
>> - add varlen_cdb and varlen_cdb_len to hold a large user cdb
>>   if needed. They start as empty. Allocation of buffer must
>>   be done by user and held until request execution is done.
>> - Since there can be either a fix_length command up to 16 bytes
>>   or a variable_length, larger then 16 bytes, commands but never
>>   both, we hold the two types in a union to save space. The
>>   presence of varlen_cdb_len and cmd_len==0 signals a varlen_cdb
>>   mode.
>> - Please use added rq_{set,get}_varlen_cdb() to set every thing
>>   up in a way that will not confuse drivers that do not support
>>   varlen_cdb's.
>> - Note that this patch does not add any size to struct request
>>   since the unsigned cmd_len is split here to 2 ushorts, which
>>   is more then enough.
> 
> My first observation is that the block stuff all looks a bit
> misnamed ... it's not about variable length commands it's about getting
> block to accept packet commands larger than BLK_MAX_CDB, be they SCSI
> OSD commands or ATA taskfiles.
> 
>> Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
>> ---
>>  block/blk-core.c       |    2 ++
>>  include/linux/blkdev.h |   27 +++++++++++++++++++++++++--
>>  2 files changed, 27 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 2a438a9..70d32f7 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -141,6 +141,8 @@ void rq_init(struct request_queue *q, struct request *rq)
>>  	rq->end_io = NULL;
>>  	rq->end_io_data = NULL;
>>  	rq->next_rq = NULL;
>> +	rq->varlen_cdb_len = 0;
>> +	rq->varlen_cdb = NULL;
> 
> possibly s/varlen/large/


If we change any names, just please do not use "large".
I'd prefer ext_cdb, where "ext" can stand for "extended" as well
as "external" (this chimes well with iSCSI naming BTW).

> 
>>  }
>>  
>>  static void req_bio_endio(struct request *rq, struct bio *bio,
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 6f79d40..f829571 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -213,8 +213,15 @@ struct request {
>>  	/*
>>  	 * when request is used as a packet command carrier
>>  	 */
>> -	unsigned int cmd_len;
>> -	unsigned char cmd[BLK_MAX_CDB];
>> +	unsigned short cmd_len;
>> +	unsigned short varlen_cdb_len;  /* length of varlen_cdb buffer */
>> +	union {
>> +		unsigned char cmd[BLK_MAX_CDB];
>> +		unsigned char *varlen_cdb;/* an optional variable-length cdb.
>> +	                                   * points to a user buffer that must
>> +	                                   * be valid until end of request
>> +	                                   */
> 
> again s/varlen/large/
> 
>> +	};
>>  
>>  	unsigned int data_len;
>>  	unsigned int extra_len;	/* length of alignment and padding */
>> @@ -484,6 +491,22 @@ enum {
>>  #define rq_is_sync(rq)		(rq_data_dir((rq)) == READ || (rq)->cmd_flags & REQ_RW_SYNC)
>>  #define rq_is_meta(rq)		((rq)->cmd_flags & REQ_RW_META)
>>  
>> +static inline void rq_set_varlen_cdb(struct request *rq, u8 *cdb, short cdb_len)
>> +{
>> +	rq->cmd_len = 0; /* Make sure legacy drivers don't get confused */
>> +	rq->varlen_cdb_len = cdb_len;
>> +	rq->varlen_cdb = cdb;
>> +}
>> +
>> +/* If ! a varlen_cdb than return NULL */
>> +static inline u8 *rq_get_varlen_cdb(struct request *rq)
>> +{
>> +	if (!rq->cmd_len && rq->varlen_cdb_len)
>> +		return rq->varlen_cdb;
>> +	else
>> +		return NULL;
>> +}
> 
> These accessors look wrong ... I think what we want is rq_get_cdb and
> rq_set_cdb that handles both <= BLK_MAX_CDB by simply a memcpy and >
> BLK_MAX_CDB by setting up the large_cdb pointer.  That way we can
> accessorize the whole lot and the drivers don't have to care about what
> BLK_MAX_CDB is set to (it simply becomes an internal convenience of
> block ... and possibly allows us to eliminate the request carries
> command data entirely).

That's seems like a terrific idea.

Benny

> 
>>  static inline int blk_queue_full(struct request_queue *q, int rw)
>>  {
>>  	if (rw == READ)
> 
> James
> 
> 
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